Compatibility information had been updated yesterday, seemingly after you post, so you likely need to 'Find Updates'. As before you might want to add a boolean extensions.checkCompatibility to about:config.

Once again with the next update FF 2.0.0.6 the add-on is not compatible. Wouldn't it be possible to set up a switch so that it fits with all future versions of FF 2.0? All my other add-ons don't have that problem. How come?

Other extensions claim support with unreleased version which I feel is wrong (as previously stated in the <a href="http://forums.mozillazine.org/viewtopic.php?p=2206294#2206294">undoclosetab thread</a>). If you don't care about the forced compatibility checking, simply disable it...